Not just writing this to piss any Fan boys off, Halo 3 isn't much of anything anymore.
While the graphics are great, the sound is horrible. Halo 3 once again brings back the weak weaponry and poor sound effect for most of them. The popular assault rifle from the first game no longer has the powerful blast effect but instead it sounds like the weak SMG guns. The shot gun also proves quite useless in Legendary and even Heroic at times. Since one shot from anything will automatically kill you dead in a heart beat and there is no way to avoid getting hit in the head by a jackels on Legendary mode.
As far as diversity goes, Halo 3 offers a wide range of weapons and vehicles. Unfortunately you will still spend more time picking up alien guns than human. Not only that, you rarely get to fly the air vehicles. Halo 3 doesn't play like a single player campaign game anymore. The entire game feels like its only for those who want to play against each other on live. Seeing how human fire arms are rarely present in the main game. Especially the big weapons you could easily spot in the first Halo which is still the better game out of the series despite the lame levels.
I have to say that Halo 3 is basically the same as the second game. Same sound from the weapons, explosions are still light and not loud or big enough. You fight mostly brutes, and they're so lame in legendary because you have to knock off their stupid helments just to kill them which means you will unload about half of your rounds on one guy before you can kill him.
The ability to play with four players in story mode is a great feature, and will make Legendary less harder if everyone is aiming at the same guy or just really good with their strategy on how to avoid getting killed too easily.
Other than that, the game is still the same boring game where your controlling a guy who never reveals his face let alone does he even talk that much.
And Cortana will pop up on the screen on almost every stage. After awhile it gets annoying. Once you do complete all the difficulties there is no reason to really play the game on campaign anymore.